This invention relates to an adherable film or sheet of a crystalline polyester coated on at least one side thereof with a carboxylated polyester primer composition having good adherability both to the base polyester and to other coating compositions such as, for example, solutions or dispersions of inks or pigments, polymer latices, dispersions of magnetic or photographic materials, and evaporatively deposited metal. These carboxylated copolyester primer compositions may also be used to impart to the crystalline polyester substrate heat sealability to itself and to other materials.
Further, this invention relates to film-forming carboxylated polyester emulsions useful in preparing primer coatings for such applications.
Polyester films or sheets, especially those of polyethylene terephthalate (PET), are widely used in such applications as packaging, decorative articles, magnetic recording media, and photographic film. In such applications, the polyester film is printed or coated with a variety of materials, which must be applied as a uniform and continuous layer and must well adhere to the base polymer. However, PET and similar polymers do not have good receptivity to most surface-treating compositions. This problem has long been recognized in the prior art, and various methods have been developed to increase the polyester film's surface receptivity. A good discussion of the prior art in this area can be found in U.S. Pat. No. 4,467,189 (Posey et al., American Hoechst Corp.) granted Oct. 9, 1984. Nevertheless, most primers used commercially are oriented to specific end uses and are suitable only for either hydrophobic or hydrophilic surface treatment. Further, most such primer compositions interfere with recycling of the polyester scrap.
U.S. Pat. No. 4,476,189 discloses a primed water-dispersible copolyester of terephthalic acid, an aliphatic dicarboxylic acid, and an aromatic sulfonated monomer with an alkylene glycol. The resultant primed polyester film is said to provide excellent adhesion to many subsequently applied aqueous or solvent-based coatings.
The presence of the aromatic sulfonated monomer, which can be sodium sulfoterephthalic acid or a similar sulfonated derivative of a phthalic acid, is a shortcoming because such starting materials are not widely commercially available and therefore are expensive; and also, PET film coated with such a primer composition would not be readily recyclable for use in contact with food because the sulfonated monomer is not FDA-approved.
It is thus desirable to provide a primed polyester film or sheet which could be manufactured from inexpensive, readily available materials, and which could be recycled for a variety of uses, thus providing an additional manufacturing advantage.